Valves that block an inlet passage, block an outlet passage and open a vent passage when in a first state and open an inlet passage, open an outlet passage and block a vent passage when in a second state are sometimes referred to as double block and bleed valves. One application of double block and bleed valves is to select one of a plurality of sample streams that is routed to a process analyzer.
It is common practice to utilize a single automated process analyzer for analyzing multiple sample streams. This significantly reduces the cost of analyzing gas and liquid process streams in petrochemical plants, refineries and other process-related industries. The sample streams are generally transported to an automated valving manifold near an analyzer by tubing or piping. The automated valving manifold is usually electronically controlled to sequentially select and divert individual sample streams to the automated analyzer.
Air actuators are used to operate valves, such as double block and bleed valves. Typical air actuators include an actuator housing that defines a cylinder, a piston disposed in the cylinder, and a cap that closes the piston in the cylinder and limits axial movement of the piston in the cylinder. Air is selectively provided into the cylinder to move the piston against a biasing force of a spring. Some air actuators include a visual indicator that is screwed into the cap to indicate the position of the piston. One such visual indicator includes a plunger that is in contact with the piston. Movement of the piston moves the plunger to indicate the position of the piston. The height of the visual indicator is typically set for each individual valve.